When determining the rheology of a fluid, it is important to take the operating conditions of that fluid into consideration. Conditions such as temperature and pressure under which the fluid is utilized are important when measuring the rheological characteristics. As an example, many drilling fluids are subjected to temperatures above 400° F. and pressures greater than 10,000 psi in deep wellbores.
Conventional apparatuses for measuring the viscosity of Newtonian fluids include a cylindrical bob suspended within a concentric tubular sleeve for immersion in the fluid to be tested. The sleeve is rotated at a known velocity, causing the fluid in the annular space around the bob to drag on the suspended bob. The torque exerted on the bob provides a measure of the fluid viscosity. Typically, a stationary frame is used to suspend the bob and sleeve using ball or roller bearings. When used in harsh environments, these bearings may become pitted or gummed up, resulting in inaccurate viscosity measurements and, eventually, resulting in failure of the instrument.
In order to improve the reliability of the bearings, flexural or torsional bearings have been used. U.S. Pat. No. 4,571,988, titled “Apparatus and Method for Measuring Viscosity,” discloses the use of a cross-spring pivot (CSP) as the flexural bearing, and is hereby incorporated by reference herein. A typical CSP is an arrangement of several flat springs configured so that when rotated for small angles, the springs bend so that the deflection appears to be about an axis. In some applications, the CSP axis is coaxial with the axis of the bob of the rheometer. Flexural pivots have many advantages. They have no sliding or rolling friction nor do they have tight clearances, so their rotational properties are very consistent. The CSP acting as a torsion spring and bearing support for the bob, permitting electronic measurement of the bob's rotational displacement to determine the fluid's shear stress. Though the device disclosed in U.S. Pat. No. 4,571,988 offers certain advantages with respect to accuracy and long-term reliability in harsh environments, it nevertheless lacks the ability to measure viscoelastic fluid properties such as the shear modulus. This ability would be useful for characterizing non-Newtonian fluids.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed with the given embodiments by the scope of the appended claims.